Implantable cardioverter defibrillators (ICDs) are implanted in patients susceptible to cardiac tachyarrhythmias including atrial and ventricular tachycardias and atrial and ventricular fibrillation. Such devices typically provide cardioversion or defibrillation by delivering low voltage pacing pulses or high voltage shocks to the patient's heart of up to 800V. The ICD operates by detecting a fast heart rate or tachyarrhythmia, upon which a battery within the device housing is coupled via an inverter to a high voltage capacitor or capacitors to charge the capacitors. When the capacitors reach a desired voltage, charging is stopped. The capacitors are discharged under control of a microprocessor to provide a therapeutic shock to the patient's heart.
While transcutaneous rechargeable battery systems have been contemplated, for example as provided in U.S. Pat. No. 5,991,665 to Wang et al., such a system has never been implemented in an ICD because of the lack of an acceptable rechargeable battery and the recharging system, and the unsuitability of current battery technologies for recharging.
In addition, a battery in an ICD must be capable of high current rates needed to charge the high voltage capacitors in a short time, so that a therapeutic shock may be delivered within a short time interval after the device has detected and diagnosed a need for the shock. If the battery has an excessive internal resistance, the current flow rate will be limited, delaying capacitor charging. This may result in syncope, ischemia (oxygen starvation) of critical organs and tissues. As a general principle, the sooner the therapy can be delivered following a detected episode, the better prospects are there for the patient's health. In addition, it is believed that therapy delivered more promptly requires a lower energy therapy, allowing the conservation of the battery's energy to extend the device life before replacement is required.
Thus, ICD designers have adopted a class of low internal resistance battery chemistries such as Lithium Pentoxide of which Lithium Silver Vanadium Oxide (SVO) is a member, using one or more such cells in selected ICD applications. These provide the required rapid capacitor charging, and are generally effective over a moderately long life. However, over the life of existing devices, as SVO battery voltage diminishes, the time interval between diagnosis of an arrhythmia and completion of capacitor charging increases, so that the effective device life is limited due to the concerns noted above about delayed treatment.
To provide extended battery life, batteries of various chemistries may be recharged. As noted above, transcutaneous recharging has been contemplated for low-energy implanted devices such as pacemakers. However, the SVO batteries preferred for ICD devices for the reasons above are considered as primary cells and thus not rechargeable by the recommendation of their manufacturers. In certain rechargeable batteries, there remains an inherent concern that multiple recharging cycles may generate elongated dendrites as electrode surfaces are replated. These metal dendrites can cause shorting. Shorting in a battery will tend to deplete it rapidly, prevent subsequent recharging, and render the device inoperable. In addition, lithium clustering, which may generate a particle of lithium between an anode and a cathode in response to rapid charging without precaution, is another known risk of attempting to recharge an SVO cell.